I often hear how people from the United States pronounce "twenty" as "twonny" in movies, etc. Why? Is this an alternative pronunciation?
It's possibly a combination of the accent in some parts, though I don't know offhand where those places would be, and local 'shorthand' type pronunciations. As long as you recognise it as twenty though, in the context of the movie, there shouldn't be a problem.
I'm not a native speaker but I've been learning mostly American English for a long time now and the so-called flapped t is widely used by American English speakers in fluent speech.
That's probably what szx heard. So twenty becomes \'twɛni\ to rhyme with 'penny'. But my guess is that in really fast speech it can even become more like tw'nny resulting in something like \'twɐni\ or \'twʌni\ which can probably be transcribed as 'twonny'.
The Oxford Advanced American Dictionary:
For more information on 'flapped t' refer to Wikipedia: